The present invention relates to x-ray imaging, particularly to a grid for reducing the deleterious effects of the scatter of x-rays during imaging, and more particularly to an improved x-ray anti-scatter grid and fabrication method involving a plurality of parallel or crossed slots formed in a substrate having low x-ray opacity and coated or filled with a material having high x-ray opacity, whereby the scattered x-rays are blocked from entering the grid.
Today's mammography machines usually use an x-ray tube with a molybdenum anode. The breast is compressed between two plates to a thickness of about 5 cm on average. Exposure times are tenths of seconds, and two views at different angles through each breast are usually taken to yield four film images per exam.
The film is not exposed directly by x-rays; rather, it is exposed by visible light produced by x-ray-induced scintillation in a screen. The screen/film system offers high detection efficiency for x-rays but lower spatial resolution than film used without a screen.
When forming an x-ray image of an object, scatter of x-rays within the object causes degradation of the image. Scatter is a particularly severe problem in medical x-ray imaging, particularly in mammography due to the degrading of the image thereby increasing the difficulty of reading the mammogram by the conventional techniques.
To reduce scattered photons (x-rays) from reaching the image plane and thus to keep the contrast high, an anti-scatter device is generally used. This device, called a Bucky grid, is placed between the breast and the film/screen. These grids are designed to be relatively transparent to x-rays arriving from the direction of the source, but have lower transmission for x-rays entering from other angles. Since scattered x-rays come from all directions, the purpose of the grid is to selectively attenuate scatter. The conventional grid consists of venetian-blind-like slats that pass most unscattered photons but block those coming from off-angles. The performance of the conventional grid is poor because the grid blocks some unscattered x-rays and nearly all of the scattered ones. This results in necessitating increase of the radiation dose to a patient by a factor of about three to produce a properly exposed film.
The grids are usually put into motion during the imaging process so as to blur the shadow of the grid itself. Problems with present designs include complex fabrication methods, poor product yield, high expense, and poor performance. The high mass of the grids requires high forces to induce rapid motion. This results in vibration of the surrounding equipment and introduces motion blur in the image. Thus, there exists a need for an x-ray anti-scatter grid that will improve the performance of x-ray imaging, particularly traditional screening mammography.
The present invention is directed to an improved x-ray anti-scatter grid, and fabrication method therefor, which overcomes the above-referenced problems with the conventionally utilized Bucky grids. The grid of this invention utilizes parallel or crossed slots of microchannels in a substrate, such as silicon, which are coated with a material having a high x-ray opacity such as gold, or filled with an appropriate liquid, such as mercury. Also, this grid eliminates the need to put it in motion during the exposure, as is required from the conventionally used grids.